custom-type-p?

[Drew Adams]

Is there a predicate that tests whether a sexp is valid as a custom type,
that is, as the value of :type in defcustom?

Looking at the code of `custom-declare-variable' (used by defcustom), it
appears that you can provide any value for :type and it is accepted. That
doesn't seem to jibe with the Elisp manual, which states that types are
lists with certain characteristics or else simple-type symbols that
abbreviate such lists. The doc gives the impression that only certain values
are acceptable as types, but I don't see where that is actually enforced.

Shouldn't there be such enforcement somewhere? Shouldn't there be a
`custom-type-p' predicate? And what happens today if you do use an "invalid"
:type? Where, if anywhere, does that have a consequence?